Cathode arrangement for electron discharge devices



Sept; 11, 1951 THOMSON ETAL 2,567,624

CATHODE ARRANGEMENT FOR ELECTRON DISCHARGE DEVICES Filed Sept. 2, 1947 2Sheets-Sheet 1 Sept. 11, 1951 CATHODE ARRANGEMENT FOR ELECTRON DISCHARGEDEVICES -Filed Sept. 2, 1947 2 Sheets-Sheet 2 5f \r 50 25 y 7/ QTTORNFYiaten ted Sept.

(BATH-ODE ARRANGEMENT FOR ELECTRON DISCHARGE DEVICES Amnf strnximmeThomson, George Paul Wright, and Alexander Rizo Rangabe,

Baldock, England AbplicationSeptember 2, 1947,-Serial 0. 771,734 InGreat Britain September 3, 1946 16 Claims.

This invention relates to electron discharge de vices and moreparticularly to cathode's'of'magnetrons. In prior artmagnetrons'serioiis difliculties have been encountered in'centri'rig thecathode, in preventing power loss due to the ca pacity effect betweenthe cathode and its adjacent filamentary heater coil, in preventingoverheating oi the filamentary heatercoil during both manufactureandinormal oper'ationof the magnetron, and in delivering at all timesthe proper amount of neat to the'cathode. This in'vention has for its"primary object the provision of an improved cathode. Other objects areto overcome the above mentioneddifiiculties. Still other objectsWill-become apparent to' those skilled in the'a'rt as thisdescription'proceeds.

Prior art magnetrons have employed a centrically located oxide-coatedcylinder heated by an adjacent filamentary heater. For best results thiscylinder should be accurately positioned'along the axis of the centralcavity of the magnetron and difiiculties in centering the cylinder havebeen encountered.

Our invention employs a large rigid' metalllc cantilever support for oneend of the cathode. This large support accurately supports the oathodeand overcomes the centering difiiculties here tofore encountered and;also serves to overcome another difficulty encountered in prior artcons't'ruction's in that it conducts excess heat away from'the cathode.H I p 7} Since prior art magnetrons deliver considerable power, theyhave large filamentary heaters, Furthermore they often are called uponto deliver out put currents which have sharp peaks, or are otherwisemodulated at high frequency. Inview of the large size of the filamentaryheater and its closeproximity to the cathode, there is a capacitiveeffect between the filamentary heater and'cathode which permitshighifrequ'ency power tojpass from the cathode to the filamentary heaterand thence to ground. The power transformer that energizes thefilamentary heater does jnot stop this fiow of current since radiofrequency energy in the secondary of the transformer easily passes tothe primaiy winding in view of the capacity between such primary andsecondary v fIfhe high frequency power passing to the primary winding isfed to ground, and since the anode is' grounded there is' an 'efie'ctiveloss of power. 7 ,our invention reduces this powerloss in that thefilamentary heater is replaced with an electron gun positioned somedistance away fromtlie cathode. The cathode'is provided with a siii "bIe2 and the bombardment of this target "creates enough heat to operate thecathode. By reason of this arrangement, the capacity eiiect tending totransfer power to the filamentary heater-is re duced. Furthermore, moreheat can beisupplied to the cathode than heretofore since heating of theprior art cathodes was limited to. thetemperature that the filamentaryheater could withstand. During the process of manufacture of a magnetronit is desirable to heat the cathode to temperatures higher thanfilamentaryheaters could withstand. Such heating, during'processiiig, isfor the purpose of converting the carbonates oi' such metals as bariumand strontium into oxides; and when a thorium oxide'cathode is used itserves to outgas the cathode member. At times, during operation of themagnetron'as a radio frequency oscillator, back-bombardment of thecathode tndsto overheat it and destroy ordistort the filamentary heater.With our arrange emtm filamentary heater of theelectron gun is notsubjected to the high temperature of the cathode,

and hence there is no damage to our device dur:

ing back-bombardment of the cathode or during processing of the cathode.v v

The electron gun referred to above asv one feature of our invention, is,together with its as-v sociated target, located in a-separate evacuatedenvelope from the one surrounding the cathode. This prevents poisoningof the gun-electrode during outgassing of the magnetron.

In the-drawings: H u

F i g;ure -1 is ,a longitudinal sectional view of -a magnetron embodyingcertain features of the nv n on; n -Fi gure 2 is a circuitdiagramshowing the electrical connections of certain parts of themagnetron to external current-sources.

Figure 3 illustrates the principleof operation of .one type-of magnetronto which the invention maybe applied. 7

, InFigure 3, which illustratesone form of magnetron to which ourinvention maybe applied, theanode blocl; H) has a plurality of resonatorcavities-38 disposed around and opening-into a maincentral cavity 39.The cathode cylinder I l is in the center of the maincavity and magneticis applied parallel with and along the axis ofthe cathode.Electronsleaving the cathode toward the'po'sitively chargedanodeare'defiected from a radial path to a curvilinear path by reas onof the magnetic flux. As theelectrons curve graze the pole pieces 40which separate the resonator cavities, they become bunched and setuposoilla'tions in the resonator cavities. Power may be extracted from theentire system by a wave guide or other coupling associated with oneresonator cavity 38. Magnetrons of this type are now well known and aremore fully disclosed in the copending application of John T. Randall andHenry A. H. Boot, Serial No. 407,680, filed August 20, 1941, whichissued on February 20, 1951, as Patent No. 2,542,966, as well as in thecopending application of James Sayers, Serial No. 577,067, filedFebruary 9, 1945, which issued on March 27, 1951, as Patent No.2,546,870.

In Figure 1, cathode member I l is shown located in the central cavity39. A copper shell l2 provided with cooling fins I3 is located incontact with and surrounding the anode ID. A power output channel [4, ofany conventional type, may be employed, to extract power from themagnetron. Pole pieces l5 and N5 of the main magnet of the magnetron areprovided to apply a magnetic flux parallel to the axis of the cathode II. These pole pieces are contiguous with the shell l2. A glass envelopeportion 11, which is sealed at pip l8 maintains the cathode spaceevacuated. Another glass envelope portion 36 also serves to maintain thecathode space evacuated.

The cathode member II is accurately centered along the axis of thecentral cavity by the heavy metallic rigid support 20 which is in turnsupported by the heavy copper cylinder 2|. This large rigid cantilevergreater accuracy than prior art cathode mounting arrangements, and hasthe further advantage of serving to hold the cathode in its properposition at all times, there being no warping or bending of the heavysupport 20. Support 26 may be composed of any suitable metal, but ispreferably either nickel or molybdenum.

During back-bombardment of the cathode during operation of themagnetron, the cathode II is highly heated. The heavy supportingcylinder 26 tends to conduct excess heat away from the cathode l I. Theexcess heat conducted away from the cathode by metallic member 20 istransferred to copper cylinder 2| where it may be dissipated by forcedair or water cooling. Cyldrical shield 23 to electrostatically focus theelectrons upon the target 29 in the well known manner.

The electron gun 22 is located in an evacuated space distinct from theone surrounding cathode ll. electron gun 22 is defined by the target 29at the left, then by the inner walls of copper cone 26, then by theinside wall of copper cylinder 2|, and finally by the glass sealingmember 31. The evacuated space surrounding the cathode H is defined atthe left by glass envelope portion N, then by the inside wall of polepiece 55, then by copper sealing member [2, then by the inner wall ofpole piece It as well as by the outer wall of cone 20 and finally by theglass envelope portion 36 as well as by the outer wall of coppercylinder 2|. By reason of the location of the cathode II and theelectron gun 23 in separate evacuated spaces, the filament of theelectron gun is not poisoned by any gases given ofi" by the cathode l lduring processing or operation thereof.

Referring to Figure 2, a power source 28 feeds primary winding 21 of astep-down transformer,

the secondary winding 26 of which supplies cursupport enables much Theevacuated space Surrounding rent to the filamentary coil 50 of electrongun 22. In order to provide high voltage to effect bombardment of thetarget 29, a source 35 of high voltage direct current has its negativepole connected to gun 22 and its positive pole connected to the metallicanode [0. The anode It! is in turn connected electrically to the target29 through the secondary Winding 32 of a pulse transformer 3| which willhereinafter be more fully described. It is obvious that the structurethus far described is a complete cathode heating arrangement for byproperly adjusting the voltage on shield 23 and the voltage of source35, the electrons can be caused to bombard the target 29 to any desiredextent. During processing of the tube, the voltages can be raised so asto effect large quantities of heat. When the tube is in operation,whereby some selfheating of the cathode occurs, the voltages can bereduced or stopped altogether and the heavy metallic cone 2!] reliedupon to conduct excess heat away.

In Figure 2, there is shown a pulse generator 36 feeding the primary 33of pulse transformer 3|. Pulse generator 30 may be any form of modulatorknown to the art, but is here referred to particularly as a generator ofvery short pulses, each pulse being of duration of the order of amicrosecond in duration. The pulse generator 39 has a repetition ratebetween several hundred and several thousand pulses per second,depending on the particular requirements of the system. The shortduration of the pulses tends to create high frequencies which willreadily pass, due to capacitive effects, between adjacent objects. Withprior art magnetrons, large quantities of energy were lost due totransfer of energy by capacity from the cathode to the filamentaryheater, and from that heater to ground through the capacitiveeffectsbetween the primary and secondary of the filament transformer.With ourconstruction, the filament 40 of the electron gun 22 is locatedat a position so removed from the cathode Ii that the capacity betweenthe cathode H and the filament 40 is negligible.

It will be seen that the electromotive force of the secondary 32, duringthe delivery of a pulse, opposes the high voltage existing across source35 and thereby stops the bombardment of target 29 for the duration ofthe pulse. This is not a disadvantage since the heavy copper memberstores suificient heat as to iron out these very slight irregularities,and moreover the time duration of the pulses is often short as comparedwith the spaces between pulses.

It is apparent that the invention has its greatest advantages andnovelty when applied to magnetrons, but some of the features of thisinvention have utility in connection with other types of electrondischarge devices. We desire it understood that the claims are notlimited to magnetrons except as they are by their terms so limited.

We emphasise that it is important, to achieve successful operation ofthe electron gun heating of a cathode, that the intense magnetic fieldnecessary for operation of the magnetron shall be symmetrical around thecathode and shall not be so disposed as to unwantedly deflect or inhibitthe electron stream intended to strike the cathode or the targetconnected thereto. We have found that it is desirable to analyse themagnetic field around, and in particular rearwardly of, the cathode orits target, and to adaeeam support for said cathode, and means adjacentsaid support for heating the cantilever "support to thereby heat thecathode.

2. A magnetron "comprising, an anode block defining a cavity, a cathodewithin said cavity, a support for the cathodeln heat 't'rans'ferrelationtherewith, and means "adj acent "said support for bombarding saidsupport with eiectrons "to thereby heat the cathode.

3. A cathode element comprising an electron emitting member, a"cantilever supporting said member in good heat transfer relation withsaid member, and means adjacent said cantilever for bombarding saidcantilever with electrons to thereby heat said electron emitting member.7

4. A cathode element for electron tubes comprising an electron emittingmember, a cantilever metallic support forsaid'niember in good heattransfer relation therewith, means adjacent said support forheating-said support to thereby heat the emitting member, -and metallicheat dissipating means of large area and in good metallic heat transferc'on'tact'with the support to dissipate excessive heat.

5. An electron discharge devicecomprising an evacuated envelope having acathode therein, a metallic cantilever support extending through theenvelope and constituting-the sole support for said cathode, saidsupporthaving a large solid cross-section to effect good heat conduction, andmeans located'o'utside'of said envelope "for applying heat to a limitedportion of said support to thereby heat the cathode by-con'duction ofheat from said limited portion, said means constituting the sole meansfor initiating heating of the cathode.

6. A magnetron comprising an anode block having a central cavity, acathode located in said cavity, a metallic heat transfer member attachedto said cathode, said heat transfer member including an electron target,an electron gun aligned with said target for bombarding said target, amodulator having its negative pole connected to said cathode and itspositive pole to said anode, and a high voltage power supply for theelectron gun having its negative pole connected to the electron gun andthe positive pole connected to said anode.

7. A magnetron comprising an anode block defining a central cylindricalcavity and a plurality of resonator cavities around the central cavityand opening into the central cavity, an oxide-coated cylindrical cathodemember, means constituting the sole support for said cathode membercomprising a cantilever metallic supporting member with the suspendedend attached to and in good heat transfer relation with the cathodemember, said supporting member positioning said. cathode member withinand concentric with the central cavity, magnet pole pieces on oppositesides of said anode block for establishing a magnetic field parallel tothe axis of the central cavity, one of said pole pieces defining anopening through which said supporting member passes, an evacuatedenvelope surrounding said central cavity, said envelope init'y, saidcantilever support constituting the'o'nly eluding a portion of saidsupporting member, another portion of said supporting member beingoutside said evacuated envelope and forming an electron target, anelectron gun separated from said target a suificient distance to avoidlosses from capacitive effects, means connected to said target and gunfor polarizing said target and "gun to efiect bombardment of the targetwith "consequent heating of said cathode, and-an evacuated envelopesurrounding the electron gun "and target.

8. A magnetron comprising an anode block defining a central cylindricalcavity and a plurality of resonator cavities around the 'rnain cavityand opening into the main cavity, magnet pole pieces adjacent oppositeendsof the-central cavity for setting up a field in the central cavityparallel to the axis thereof, one of said magnet poles having an openingtherethrough "aligned with said central cavity, a cylindricaloxidecoated cathode member, means supportingsaid cathode member in'saidcentral cavity and concentric therewith, said means constituting thesole support for said cathode member and consisting of a tubular memberwith its axis an'extension of that of the cathode member, said tu' bularmember passing through said opening in said pole piece, an evacuatedenvelope'surrou'n'ding the central cavity and sealed to the outer wallof said tubular member, saidtubular member being sealed at the first endthereof'which supports the cathode member, a sealing member at thesecond end of said tubular member "to maintain the same evacuated, anelectron gun located within the tubular member, pulsing meanselectrically coupled to said anode and cathode for applying pulsepotentials between said cathode and anode, means electrically coupled tosaid electron gun and-said anode'for supplying high voltage potential'tooperate the'electron gun, means adjacent said electron gun to focus theoutput of the electron gun upon an inner wall of said tubular membernearest the cathode member whereby to heat the cathode, and coolingmeans associated with the second end of said tubular member.

9. A cathode structure comprising an elongated metallic member a firstend of which is an electron emitting surface, said member defining anopening throughout substantially the remainder of the elongated member,said first end sealing the first end of said member, means sealing thesecond end of said member, an electron gun Within said opening forbombarding the inside of said member at a point near said surface tothereby heat said surface, means sup porting said member only atpositions remote from said first end, said supporting means constitutingthe sole support for said member, an anode adjacent said cathode, and anenvelope surrounding said cathode and anode, said envelope being sealedaround the outer wall of said tubular member.

10. A magnetron as defined in claim 1 including a first evacuatedenvelope surrounding the heating means and the portion of said supportheated thereby; and a second evacuated envelope surrounding theremainder of said support as well as surrounding the anode block and thecathode.

11. A cathode element comprising an electron emitting member, acantilever in good heat transfer relation with said member andsupporting the latter, electron bombarding means for bombarding saidcantilever with electrons to thereby heat said emitting member, saidcantilever being tubular and containing the electron bombarding meanstherein and sealing means for rendering the interior of said cantileverairtight.

12. A magnetron comprising an anode block defining a cavity, a cathodewithin said cavity, a support for the cathode in heat transfer relationtherewith, and an electron gun including an electron emitter forbombarding said support with electrons to thereby heat the cathode, saidelectron gun having its electron emitter substantially spaced from saidsupport to thereby efiect low capacity between the electron emitter andsaid cathode.

13. A cathode element as defined in claim 3 in which said bombardingmeans comprises an electron emitter substantially spaced from saidsupport to thereby efieot low capacity between the last-named electronemitter and the cathode.

14. A cathode structure comprising a unitary member of good thermalconductivity, electron emitting means on a limited portion of saidmember near a first end thereof, means which constitutes the solesupport for said member and is attached to it remote from said firstend, and means adjacent said member for heating the member between saidlimited portion and the second end thereof, said last-named meansincluding a heat generator of suflicient capacity to raise thetemperature of the electron emitting means to electron emissiontemperature.

15. A cathode element comprising an electron emitting member, a supportfor said member, said support being contiguous with said emitting memberand in good heat transfer relation therewith, said support beingcomposed of a material having good thermal conductivity, and meansadjacent said support for bombarding said support with electrons tothereby heat said support and in turn heat said electron emittingmember, said last named means including a source of electrons and meansfor accelerating and directing electrons derived from said source uponsaid support.

16. A cathode element comprising an elongated structural member, anelectron emitting surface formed upon said structural member, saidstructural member being composed of a material having good thermalconductivity and being in good heat transfer relation with said electronemitting surface, and means adjacent said structural member forbombarding said member with electrons to thereby heat said member and inturn heat said electron emitting surface, said last named meansincluding a source of electrons spaced from said elongated structuralmember and also including means for accelerating and directing electronsderived from said source upon a portion of said structural member.

AMHERST FELIX HOME THOMSON. GEORGE PAUL WRIGHT. ALEXANDER RIZO RANGABE.

REFERENCES CITED The following references are of record in the file ofthis patent:

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